1. Field of the Invention
The present invention relates to a Doherty microwave amplifier (hereinafter, referred to as Doherty amplifier) and a signal processing method thereof.
2. Description of the Related Art
A Doherty amplifier is well known as an amplifier with high efficiency. The Doherty amplifier equipped with a main amplifier to perform linear amplification and a peaking amplifier to compensate deterioration in linearity of the main amplifier in a higher output area. Since the Doherty amplifier can obtain a large output by low power, it has been used even in a high-frequency area such as a microwave (refer to, for example, R. J. McMorrow, D. M. Voton, and P. R. Malonney “The Doherty microwave amplifier”, 1994, IEEE MTTS Digest, TH3-E, 1994).
FIG. 1 is an exemplary block diagram showing an existing Doherty amplifier. The Doherty amplifier in FIG. 1 has a main amplifier 100, a peaking amplifier 200, an output load 300, a distributor 400 and a bias controller 500. The distributor 400 distributes a signal which is input to the Doherty amplifier into the main amplifier 100 and the peaking amplifier 200.
FIG. 2 is an exemplary view showing an input/output characteristic of the Doherty amplifier in FIG. 1. FIG. 2 shows an operation line ML of input to output of the main amplifier 100 and an operation line PL of input to output of the peaking amplifier 200. The peaking amplifier 200 starts its output from an S point at which the saturation of the output from the main amplifier 100 starts.
The bias controller 500 checks an input signal level of the Doherty amplifier to output a bias control voltage to the peaking amplifier 200 when the input signal level exceeds the input level at the S point. The peaking amplifier 200 compensates the deterioration in output from the main amplifier 100 to output an amplified signal to the output load 300. As a result, as shown with a dot line part in FIG. 2, a linear signal output combined with the output from the main amplifier 100 can be obtained at the output load 300 [refer to, for example, Jpn. Pat. Appln. KPKAI Publication No. 2004-173231 (eighth page, FIG. 2)].
However, in a configuration in FIG. 1, the input level at the S point from which the peaking amplifier 200 starts its operation is a fixed level in advance. Therefore, the changes in the input level from which the gains of the main amplifier 100 and the peaking amplifier 200 and the saturation of the main amplifier 100 start by the temperature make the existing amplifier impossible to accurately perform a linear compensation. And the existing Doherty amplifier becomes impossible to accurately perform the linear compensation also depending on the individual difference in the main amplifier 100.
The existing Doherty amplifier monitors an input signal level thereof and regulates a voltage of a bias signal to control an output from a peaking amplifier. Therefore, influence of a change in temperature or individual difference in Doherty amplifiers causes the problem that accurate linearity can not be obtained.